a. Field of the Invention
The present invention relates to an improvement in the method of producing uniform molded articles from a mixture of a synthetic resin and a cross-linking agent, as well as a molding apparatus pertaining to said method.
B. Description of the Prior Art
Cross-linking of a polyolefin resin, such as polyethylene, is usually performed by a procedure whereby a mixture obtained by uniformly dispersing a cross-linking agent in said polyethylene is first made into molded articles by means of an extrusion-molding machine, followed by a cross-linking process. As widely used known cross-linking processes, there are the high-pressure steam process, the high-temperature liquid process, the lead-encasing vulcanization process, the Grace's process, the radiation process, etc.; each process having merits and demerits. In addition, there has recently been developed a new cross-linking process called Engel's process. According to Engel's process, a cross-linking agent is mixed with a thermoplastic material within a screw-driven extruding machine, the resulting mixture is introduced into a pressure chamber to be compressed under a pressure of 2000 Kg/cm.sup.2 instantaneously to effect uniform dispersion of said cross-linking agent without giving rise to the cross-linking reaction, and then the thus treated mixture is passed through a die held at a temperature high enough to effect the cross-linking reaction to thereby accomplish the desired cross-linking. An object of this process is to achieve uniform dispersion of the cross-linking agent by means of said pressure chamber, and cross-linking by heating from the outside at the time of passing the mixture through said die. It is technically based on the conception that said thermoplastic material acquires a heat curing property after completion of the cross-linking reaction and is no longer suitable for molding or drawing. Consequently, molding by this process must be performed just before the material becomes unsuitable for molding or drawing and is therefore very unstable.